A general method of gaining entry into the rare aldoses has been sought for over 50 years with little apparent success but with little diminution in the desirability of or motivation for finding and developing appropriate chemical procedures. One approach which has been explored with mixed success is the oxidation of alditols to the corresponding aldoses of the same carbon number. By "alditol" is meant a polyhydroxylic compound of general formula HOCH.sub.2 (CHOH).sub.n CH.sub.2 OH, sometimes referred to as a sugar alcohol. Of this class the tetritols (n=2), pentitols (n=3), and hexitols (n=4) are the most important representatives. The corresponding aldoses have the formula HOCH.sub.2 (CHOH).sub.n CHO, where n=2 corresponds to the tetroses, n=3 to the pentoses, and n=4 to the hexoses. Glattfeld and Gershon made several pertinent observations in their investigation of the oxidation of mannitol and galactitol (dulcitol) [J. Am Chem. Soc., 60, 2013 (1938)]. From a preliminary study of the oxidation of mannose they concluded that the operative reaction sequence was EQU mannose.fwdarw.mannonic acid.fwdarw.mannuronic acid.
Using platinum oxide to oxidize mannitol they observed that platinum oxide was reduced to zerovalent platinum accompanied by the stoichiometric oxidation of mannitol. The zerovalent platinum then catalyzed the oxidation of mannitol by oxygen. The oxidation of mannitol afford major amounts of carboxylic acids (more than about 1/3 of the product) and the investigators proposed a squence EQU mannitol.fwdarw.mannose.fwdarw.mannonic acid.fwdarw.mannuronic acid.
Generalizing from the above one may then write the sequence, ##STR1## from which one can draw several conclusions. The conversion of II (aldose) to III (aldonic acid) implies that under the reaction conditions the aldehydric functional group, CHO, is oxidized faster than is the primary hydroxyl group, CH.sub.2 OH. The fact that both II and II+IV are present in the reaction mixture implies that the oxidation of CH.sub.2 OH is at least competitive with that of CHO, for if CHO were oxidized much faster than CH.sub.2 OH then at least II and perhaps IV also should be minor products, contrary to what was observed.
Somewhat later K. Heyns and H. Paulsen, Ang. Chem. 69, 600 (1957) used zerovalent platinum supported on charcoal as a catalyst in the oxidation of alditols by oxygen. These workers proceeded on the basis that in the oxidation the rate determining step was dehydrogenation, and oxygen merely served as an acceptor for "activated hydrogen" to remove the latter from equilibrium through formation of water. This seemed to be confirmed by their report that the use of higher oxygen pressure offered no advantage. They also reported that primary alcohol groups are generally oxidized in neutral to slightly acidic solution as far as the aldehyde stage and only to a very slight degree further to the acid. However, aldehyde yields were not always satisfactory. These same workers also stated that the aldehyde group of aldoses can be readily oxidized to their carboxylic acids.
Almost contemporaneously K. Heyns and M. Beck, Chem. Ber., 91, 1720 (1958) described the preparation of L-gulose by the oxidation of sorbitol using zerovalent platinum supported on carbon. They noted that the medium had a profound effect on the course of oxidation, consistent with their prior observation that oxidation of erythritol in glacial acetic acid afforded only reducing reaction products with no acid formation, and with decreasing acetic acid content in aqueous systems the yields of reducing substances and byproducts increased. Similarly, in the oxidation of sorbitol the aldose yields increased significantly upon changing from 30% acetic acid to water as a reaction medium. However, the ratio of uronic acid to aldose also increased, going from a low of 0.07 in 30% acetic acid to 1.53 in water at 60.degree. C. After 8 hours, oxidation in water at 40.degree. C. afforded a total yield of only 35-8% aldoses and 10-12% uronic acids. The aldoses were shown to be D-glucose and L-gulose when D-sorbitol was the alditol. There was also an implication that the zerovalent platinum on carbon was readily poisoned.
It seems that the aforementioned teachings particularly pertinent to the present application may be fairly summarized as follows. 1. Oxidation of alditols in water at autogeneous pH as catalyzed by zerovalent platinum and effected by oxygen, affords aldoses in yields under 40% accompanied by major amounts of carboxylic acids as coproducts, with the weight ratio of acids to aldoses being generally greater than about 0.3, presumably because the oxidation of the aldehyde functionality is easier than, or competitive with, the oxidation of the hydroxymethyl group when catalyzed by zerovalent platinum. 2. These oxidations are unaffected by oxygen pressure because oxygen is not involved in the rate determining step. 3. There is some indication that zerovalent platinum has a limited lifetime as a catalyst in these oxidations, which is consistent with our observations in other unrelated work that, for example, gluconic acid and/or glucuronic acid serves as a potent poison to zerovalent platinum catalysts.
In contrast to the prior art, we have developed a method of oxidizing alditols using as a catalyst zerovalent platinum on selected supports and using oxygen at superatomspheric pressure. Our method affords aldoses in a product mixture containing less than about 20 weight percent carboxylic acids relative to the formed aldoses. To put this into perspective, in our method the acid: aldose product ratio is less than 0.2, whereas the comparable ratio found in the prior art methods seems to be at least 0.3. Furthermore, the relatively low acid:aldose product ratio is maintained even with aldose yields of 50% and better. Because our catalyst is not readily poisoned under reaction conditions, perhaps because of our relatively low acid yields, the oxidation can be effected in a continuous process. Thus our invention has the advantages of being more selective, especially with respect to carboxylic acid formation, of effecting relatively high conversion of alditol in a reasonable time, and of being capable of being performed in a continuous process, thereby making it significantly advantageously over the prior art. n comparison to the work of Heyns and Beck, we have achieved higher selectivity and higher aldose yields by effecting oxidation at superatomspheric pressure, whereas the achieved higher selectivity at a sacrifice in aldose yields by using acetic acid in the reaction medium.